Microporous films or membranes have a structure that enables fluids to flow through them or into them. The effective pore size is at least several times the mean free path of the flowing molecules, namely, from several micrometers down to about 100 Angstroms. Such sheets are generally opaque, even when made from an originally transparent material, because the surfaces and internal structure scatter visible light.
Microporous membranes or films have been utilized in a wide variety of applications, such as for the filtration of solids, for the ultrafiltration of colloidal matter, as diffusion barriers or separators in electrochemical cells, in the preparation of synthetic leather, and in the preparation of cloth laminates. The latter utilities require, of course, permeability to water vapor but not liquid water when preparing such materials as synthetic shoes, raincoats, outer wear, camping equipment such as tents, and the like. Moreover, microporous membranes or films are often utilized for microfiltration of antibiotics, beer, oils, bacteriological broths, as well as for the analysis of air, microbiological samples, intravenous fluids, vaccines, and the like. Microporous membranes or films are also utilized in the preparation of surgical dressings, bandages, and in other fluid or gas transmissive medical applications.
Microporous membranes or films may be laminated to other articles to make laminates having particular utility. Laminates are often used to provide enhanced mechanical strength or specific aesthetic character. Such laminations may include a microporous layer and an outer shell layer to provide a particularly useful garment material. Laminates including a microporous layer also have utility as imaging substrates.
Microporous membranes or films of crystallizable thermoplastic polymers such as, for example, polyolefins, polyesters and polyamides, have been prepared using solid-liquid thermally induced phase separation techniques. The polymer in this technique is melt-blended with a compatible liquid such as mineral oil or mineral spirits, is shaped and cooled under conditions to achieve thermally induced phase separation, followed by orienting, i.e., stretching, the article and, optionally, removing the compatible liquid.
Ethylene-vinyl alcohol copolymer porous membranes have been disclosed. These membranes are produced by dissolving the ethylene-vinyl alcohol copolymer in a solvent such as dimethyl sulfoxide, dimethylacetamide, methylpyrrolidone, pyrrolidone, or a combination of such solvents, and then feeding the polymer solution into a coagulation bath. The membranes and fibers formed by this method are useful for medical purposes such as hemodialysis and for treating blood plasma. Other solvents recognized as being capable of dissolving ethylene-vinyl alcohol copolymers include monohydric alcohols, polyhydric alcohols, phenol, metacresol, formic acid, and mixtures thereof with water, but these are not preferred for the purpose of producing hemodialysis membranes with desirable balanced permeabilities.
Ethylene-vinyl alcohol copolymer microporous powders and membranes have also been described as being prepared by a process utilizing phase separation by means of thermally induced triggering or triggering induced by a non-solvent of a solution of the ethylene-vinyl alcohol copolymer in a mixture containing .omega.-caprolactam as the essential dissolving constituent.